1. Field of the Invention
The present invention relates to a magnetic recording medium and, particularly, to a magnetic recording medium having excellent lubricating properties and running properties.
2. Description of the Prior Art
In magnetic recording media and, particularly, in audio cassette tapes, the tape runs at a low speed in contact with a fixed head.
Recently, since high performance characteristics are required for cassette tapes, the surface of the magnetic recording layer is smoothened to improve the contact with the head and to improve high frequency properties.
However, in prior art tapes, since the friction due to the head is high, the running of the tape grows worse with "squeaking" occurring and, in some cases, tape running stops.
Hitherto, in order to reduce the coefficient of friction of the magnetic recording layer, higher fatty acids, higher fatty acid esters, paraffin hydrocarbons or silicone oils have been added to the magnetic coating containing a ferromagnetic powder and a binder. These additives are described in Japanese Pat. Nos. 18064/66, 186/68, 669/68 and 15624/72, and U.S. Pat. Nos. 2,654,681, 3,470,021, 3,497,411, 3,525,694 and 3,634,253.
However, it is impossible to provide video tapes with sufficient lubrication properties and durability using these additives. Further, a "blooming" phenomenon easily occurs, by which these additives exude on the surface of the magnetic recording layer to cause adhesion of the tapes during storage.
Further, mineral oils, vaseline or paraffin waxes as described in U.S. Pat. No. 3,525,694 and Japanese Pat. No. (OPI) 84405/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".), can be used as lubricants.
However, the lubrication properties resulting from the use of these lubricants is still not sufficient. Further, since these lubricants have a remarkably poor compatibility with the binder, dispersion of the finely-divided ferromagnetic powder is often hindered with the squareness ratio of the magnetic recording layer decreasing as a result.
Moreover, a white powder often forms on the surface of the tapes or adhesion of the tapes often occurs because of the blooming phenomenon.